Method of making a rectangular nickel-metal hydride secondary cell

ABSTRACT

A rectangular nickel-metal hydride secondary cell permits improving the air-tightness and-also permits preventing a metal case from being deformed in the step of folding an open end portion of the metal case. The rectangular nickel-metal hydride secondary cell, comprising a rectangular cylindrical metal case having a bottom, an electrode group housed in the metal case, an alkali electrolyte contained in the metal case, a rectangular sealing plate mounted near the open end portion of the metal case; and an insulating gasket interposed in a compressed state between the inner wall near the open end of the metal case and the sealing plate, wherein the metal case comprises a folded portion formed by inwardly folding the open end portion of the metal case and an inwardly projecting stepped portion formed along the inner surface of the metal case below the folded portion, the sealing plate is fixed to the metal case at the portion between the folded portion and the stepped portion of the metal case with the insulating gasket interposed therebetween, the folded portion at a corner portion of the open end portion of the metal case has an angle of 80° to 100°.

This is a division of application Ser. No. 08/095,429, filed on Jul. 23,1993, now U.S. Pat. No. 5,372,897.

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION

The present invention relates to a rectangular nickel-metal hydridesecondary cell.

A rectangular nickel-metal hydride secondary cell permits an energydensity higher than that of a rectangular nickel-cadmium cell and, thus,is used widely nowadays. Because of the high energy density achieved bythe rectangular nickel-metal hydride secondary cell, it is necessary toensure an excellent air-tightness even in the event of heat generationcaused by, for example, over-charging.

In general, the rectangular nickel-metal hydride secondary cellcomprises a rectangular cylindrical metal case having a bottom. The openperipheral portion of the metal case is inwardly folded to form a foldedportion. Also, an inwardly projecting stepped portion is formed belowthe folded portion of the metal case. An electrode group is housed inthe metal case. The electrode group is constructed such that a nickelpositive electrode and a hydrogen absorption alloy negative electrodeare alternately superposed one upon the other with a separatorinterposed between the adjacent nickel electrode and the hydrogenabsorption alloy electrode. An alkali electrolyte is also contained inthe metal case. The secondary cell further comprises a rectangularsealing plate, which is fixed to the metal case at the position with aninsulating gasket between the folded portion and the stepped portion,and the insulating gasket formed of a synthetic resin and positionedalong the inner wall of the metal case. The gasket is interposed under acompressed state between the open end portion of the metal case and theperiphery of the sealing plate so as to hold the peripheral portion ofthe sealing plate.

In the secondary cell of the construction described above, the sealingplate and the insulating gasket are fixed to the open end portion of themetal case, as follows. In the first step, the insulating gasket holdingthe sealing plate is mounted on a stepped portion along the innersurface of the metal case. Then, the open end portion of the metal caseis pressed to reduce the diameter of the metal case in this portion,followed by inwardly folding the open end portion of the metal case soas to fix the sealing plate together with the insulating gasket to theinner surface of the metal case in a region near the open end portion.

As described above, an operation for reducing the diameter in the openend portion of the metal case is included in the process. What should benoted is that the material of the metal case in the shrunk portion isdistributed so as to be concentrated on the four corner portions in theopen end portion of the metal case, which is rectangular in crosssection. Naturally, the corner portion is rendered higher than thecentral portion of the side positioned between two adjacent cornerportions. When the open end portion having the particular shape isinwardly folded, the folding angle of the corner portion is renderedgreater than that in the central portion of the side, giving rise to aproblem that the air-tightness of the secondary cell is changeddepending on the folding angle of the corner portion. To be morespecific, if the folding angle of the corner portion is increased in anattempt to improve the air-tightness, an excessive force is applied tothe corner portion of the metal case. Therefore, the metal case occursdeformation such as deflection or strain. On the other hand, if thefolding angle of the corner portion is diminished in an attempt toprevent the metal case from being deformed, the air-tightness of thesecondary cell is markedly lowered.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a rectangularnickel-metal hydride secondary cell which permits improving theair-tightness and also permits preventing a metal case from beingdeformed in the step of folding an open end portion of the metal case.

Another object of the present invention is to provide a method ofmanufacturing a rectangular nickel-metal hydride secondary cell, whichpermits curling in a desired shape the upper end of the open end portionof the metal case after the diameter-reducing treatment, which permitsreadily removing the curling mold from the metal case, and which permitsimproving the air-tightness of the manufactured cell.

Another object is to provide a method of manufacturing a rectangularnickel-metal hydride secondary cell, which permits preventing a sealingplate from being deformed in the sealing step and also permits improvingthe air-tightness of the manufactured cell.

Still another object is to provide a method of manufacturing arectangular nickel-metal hydride cell, which permits sufficientlyimproving the discharge capacity and the discharge voltage before thecapacity screening inspection.

According to a first embodiment of the present invention, there isprovided a rectangular nickel-metal hydride secondary cell, comprising arectangular cylindrical metal case having a bottom; an electrode grouphoused in the metal case and constructed such that a nickel positiveelectrode and a hydrogen absorption alloy negative electrode arealternately superposed one upon the other with a separator interposedbetween the positive and negative electrodes; an alkali electrolytecontained in the metal case; a rectangular sealing plate mounted nearthe open end portion of the metal case; and an insulating gasket formedof a synthetic resin and interposed in a compressed state between theinner wall near the open end of the metal case and the peripheralportion of the sealing plate; wherein the metal case comprises a foldedportion formed by inwardly folding the open end portion of the metalcase and an inwardly projecting stepped portion formed along the innersurface of the metal case below the folded portion, the sealing plate isfixed to the metal case with the insulating gasket at the portionbetween the folded portion and the stepped portion of the metal case,the folded portion at a corner portion of the open end portion of themetal case has an angle of 80° to 100°, and the folded portion at theside positioned between two adjacent corners of the open end portion ofthe metal case has an angle smaller than that of the corner portion.

According to a second embodiment of the present invention, there isprovided a method of manufacturing a rectangular nickel-metal hydridesecondary cell, comprising the steps of:

housing an electrode group and an alkali electrolyte in a rectangularcylindrical metal case having a bottom and a stepped portion formedalong the inner surface of the metal case by enlarging the open endportion of the metal case, the electrode group being constructed suchthat a nickel positive electrode and a hydrogen absorption alloynegative electrode are superposed one upon the other with a separatorinterposed between the adjacent positive and negative electrodes;

mounting a rectangular cylindrical insulating gasket having a bottom anda rectangular sealing plate housed therein in advance on the steppedportion of the metal case, a rectangular hole being formed in the bottomof the insulating gasket;

reducing the diameter of the open end portion of the metal case to avalue substantially equal to that of the main body portion of the metalcase by inserting the open end portion of the metal case to a drawingmold so as to press inward the rising wall of the insulating gasket, themold having a rectangular hollow in the central portion and a taperedportion formed by outwardly enlarging the inner surface of the hollow inthe lower portion of the mold;

abutting a curling mold against the open end portion of the metal caseby dropping the curling mold to the metal case so as to inwardly foldthe open end portion of the metal case and, thus, to permit the sealingplate to be fixed under pressure to the metal case with the insulatinggasket interposed therebetween, the curling mold having a rectangularhollow in the central portion, a rectangular recess formed in the bottomportion to communicate with the hollow and sized larger than the hollow,and a tapered portion formed in the inner surface in the lower endportion of the recess, the tapered portion having an angle of 0.5° to10°; and

raising the curling mold while holding downward the sealing plate withknock-out vertically movable within the hollow of the curling mold so asto detach the curling mold from the metal case.

According to a third embodiment of the present invention, there isprovided a method of manufacturing a rectangular nickel-metal hydridesecondary cell, comprising the steps of:

housing an electrode group and an alkali electrolyte in a rectangularcylindrical metal case having a bottom and a stepped portion along theinner surface of the metal case formed by enlarging the open end portionof the metal case, the electrode group being constructed such that anickel positive electrode and a hydrogen absorption alloy negativeelectrode are superposed one upon the other with a separator interposedbetween the adjacent positive and negative electrodes;

mounting a rectangular cylindrical insulating gasket having a bottom anda rectangular sealing plate housed therein in advance on the steppedportion of the metal case, a rectangular hole being formed in the bottomof the insulating gasket, and the sealing plate having longer sidesurfaces which are curved outward;

reducing the diameter of the open end portion of the metal case to avalue substantially equal to that of the main body portion of the metalcase by using a drawing mold so as to press inwards the rising wall ofthe insulating gasket, the mold having a rectangular hollow in thecentral portion and a tapered portion formed by outwardly enlarging theinner surface of the hollow in the lower portion of the mold; and

abutting a curling mold against the open end portion of the metal caseso as to inwardly fold the open end portion of the metal case to form afolded portion and, thus, to permit the sealing plate to be fixed underpressure to the metal case with the insulating gasket interposedtherebetween, the curling mold having a rectangular hollow in thecentral portion, a rectangular recess formed in the bottom portion tocommunicate with the hollow and sized larger than the hollow, and atapered portion formed in the inner surface in the lower end portion ofthe recess;

wherein the hollow of the drawing mold has outwardly curved innersurfaces in the regions corresponding to the curved side surfaces of thesealing plate, and the recess included in the curling mold has outwardlycurved inner surfaces in the regions corresponding to the curved sidesurfaces of the sealing plate.

Further, according to a fourth embodiment of the present invention,there is provided a method of manufacturing a rectangular nickel-metalhydride secondary cell, comprising the steps of:

applying at least one cycle of charging-discharging cycles to arectangular nickel-metal hydride secondary cell including an electrodegroup and an alkali electrolyte, the electrode group being constructedsuch that a nickel positive electrode and a hydrogen absorption alloynegative electrode are superposed one upon the other with a separatorinterposed between the adjacent positive and negative electrodes;

permitting the secondary cell with a small residual capacity to standfor aging; and

fully charging the secondary cell after the aging step, followed bydischarging the secondary cell so as to perform a capacity screeninginspection of the secondary cell.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention, and together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIG. 1 is a cross sectional view showing a rectangular nickel-metalhydride secondary cell in Example 1 of the present invention;

FIG. 2 is an oblique view showing the metal case included in thesecondary cell shown in FIG. 1;

FIG. 3 is a cross sectional view showing a corner portion in the openend of the metal case included in the secondary cell shown in FIG. 1;

FIG. 4 is a cross sectional view showing a side portion positionedbetween two adjacent corner portions in the open end of the metal caseincluded in the secondary cell shown in FIG. 1;

FIG. 5 is a plan view showing a partial portion of the secondary cellshown in FIG. 1;

FIG. 6 is a graph showing the relationship between the leaving time andthe electrolyte leakage occurrence in the rectangular nickel-metalhydride secondary cell in Example 1 of the present invention;

FIG. 7 is a cross sectional view showing a partial portion of arectangular nickel-metal hydride secondary cell according to anotherembodiment of the present invention;

FIG. 8 is a plan view showing a hat-shaped terminal cap used in arectangular nickel-metal hydride secondary cell according to anotherembodiment of the present invention;

FIG. 9 is a cross sectional view along line a--a shown in FIG. 8;

FIG. 10 is a cross sectional view showing a partial portion of arectangular nickel-metal hydride secondary cell according to anotherembodiment of the present invention;

FIGS. 11A to 11I are cross sectional views collectively showing a methodof manufacturing a rectangular nickel-metal hydride secondary cell inExample 2 of the present invention;

FIG. 12 is a cross sectional view along line b--b shown in FIG. 11A, thecross sectional view showing the drawing mold;

FIG. 13 is a cross sectional view along line c--c shown in FIG. 11C, thecross sectional view showing the curling mold;

FIG. 14 is a cross sectional view showing in a magnified fashion aportion F included in FIG. 11C;

FIGS. 15A to 15I are cross sectional views collectively showing a methodof manufacturing a rectangular nickel-metal hydride secondary cell inExample 3 of the present invention;

FIG. 16 is a cross sectional view along line d--d shown in FIG. 15A, thecross sectional view showing the drawing mold;

FIG. 17 is a cross sectional view along line e--e shown in FIG. 15C, thecross sectional view showing the curling mold;

FIG. 18 is a plan view showing a group of sealing lids performing anexplosion-preventing function and acting as a terminal;

FIG. 19 is a plan view showing the secondary cell shown in FIG. 15I;

FIG. 20 is a cross sectional view showing a rectangular nickel-metalhydride secondary cell manufactured by the methods employed in Examples4 to 8 of the present invention;

FIG. 21 is a graph showing the increases in the discharge capacity ratioat the first charging-discharging cycle based on the values at the timeof the capacity screening inspection in respect of the rectangularnickel-metal hydride secondary cells manufactured in Examples 4 to 8 ofthe present invention; and

FIG. 22 is a graph showing the changes in the discharge capacity ratiorelative to the number of times of the charging-discharging cycles inrespect of the rectangular nickel-metal hydride secondary cellsmanufactured in Examples 4 to 8 of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a rectangular nickel-metal hydridesecondarycell,.comprising a rectangular cylindrical metal case having abottom; an electrode group housed in the metal case and constructed suchthat a nickel positive electrode and a hydrogen absorption alloynegative electrode are alternately superposed one upon the other with aseparator interposed between the positive and negative electrodes; analkali electrolyte contained in the metal case; a rectangular sealingplate mounted near the open end portion of the metal case; and aninsulating sealing gasket formed of a synthetic resin and interposed ina compressed state between the inner wall near the open end of the metalcase and the peripheral portion of the sealing plate; wherein the metalcase comprises a folded portion formed by inwardly folding the open endportion of the metal case and an inwardly projecting stepped portionformed along the inner surface of the metal case below the foldedportion, the sealing plate is fixed to the metal case with theinsulating gasket interposed therebetween at the position between thefolded portion and the stepped portion of the metal case, the foldedportion at a corner portion of the open end portion of the metal casehas an angle of 80° to 100°, and the folded portion at the sidepositioned between two adjacent corners of the open end portion of themetal case has an angle smaller than that of the corner portion.

In the secondary cell of the present invention, it is necessary for thefolded portion in the corner portion of the open end of the metal caseto have an angle of 80° to 100°. If the folding angle of the cornerportion is less than 80°, the air-tightness is lowered. On the otherhand, if the folding angle on the corner portion exceeds 100°, the metalcase is deformed in the step of forming the folded portion. The foldingangle in the corner portion of the open end portion noted above ispreferable fallen within a range of between 80° and 95°.

The folding angle of the folded portion in the side interposed betweentwo adjacent corner portions must be smaller than that at the cornerportion and is preferably fallen within a range of between 45° and 90°.If the folding angle of the side is smaller than 45°, the air-tightnesstends to be lowered. On the other hand, folding angle ofthe side exceeds90°, it may be difficult to prevent the metal case from being deformedin the step of forming the folded portion.

The insulating sealing gasket is preferably be made of a polyamideseries resin such as nylon 6, 6 mixed with carbon black or graphite.

The rectangular nickel-metal hydride secondary cell according to thepresent invention produces a prominent effect. Specifically, the openend portion of the metal case, which is rectangular in cross section, isinwardly folded to form a folded portion, as described previously. Whatshould be noted is that the folding angle in the corner portion of thefolded portion is set to fall within a range of between 80° and 100° inthe present invention. In addition, the folding angle in the sideinterposed between two corner portions is set to be smaller than that ofthe corner portion. The folding angles thus defined makes it possible toprevent the metal case from being deformed in the step of forming thefolded portion. To be more specific, the metal case is prevented frombearing deformation such as deflection or strain without fail.

The present invention also provides a method of manufacturing arectangularnickel-metal hydride secondary cell, comprising the steps of:

housing an electrode group and an alkali electrolyte in a rectangularcylindrical metal case having a bottom and a stepped portion formedalong the inner surface of the metal case by enlarging the open endportion of the metal case, the electrode group being constructed suchthat a nickel positive electrode and a hydrogen absorption alloynegative electrode are superposed one upon the other with a separatorinterposed between the adjacent positive and negative electrodes;

mounting a rectangular cylindrical insulating gasket having a bottom anda rectangular sealing plate housed therein in advance on the steppedportionof the metal case, a rectangular hole being formed in the bottomof the insulating gasket;

reducing the diameter of the open end portion of the metal case to avalue substantially equal to that of the main body portion of the metalcase by inserting the openend portion of the metal case to a drawingmold so as topress inwards the rising wall of the insulating gasket, themold having a rectangular hollow in the central portion and a taperedportion formed by outwardly enlarging the inner surface of the hollow inthe lower portion of the mold;

abutting a curling mold against the open end portion of the metal caseby dropping the curling mold to the metal case, so as to inwardly foldthe open end portion of the mental case and, thus, to permit the sealingplateto be fixed under pressure to the metal case with the insulatinggasket interposed therebetween, the curling mold having a rectangularhollow in the central portion, a rectangular recess formed in the bottomportion to communicate with the hollow and sized larger than the hollow,and a tapered portion formed in the inner surface in the lower endportion of the recess, the tapered portion having an angle of 0.5° to10°; and

raising the curling mold while holding downward the sealing plate withknock-out vertically movable within the hollow of the curling mold so asto detach the curling mold from the metal case.

In the method of the present invention, used is a curling mold having atapered portion formed in the inner surface in the lower portion of therecess of the mold. In the present invention, it is necessary for thetapered portion to have an angle of 0.5° to 10°, as pointed out above.If the tapering angle is less than 5°, it is difficult to detach thecurling mold from the metal case. On the other hand, if the taperingangle exceeds 10°, it is difficult to fold the open end portion of themetal case.

In order to diminish effectively the diameter in the open end portion ofthe metal case, it is desirable to form the tapered portion of thecurlingmold by outwardly expanding the inner surface in the lower endportion of the hollow by 1° to 5°.

It is also desirable to use a drawing mold and a curling mold meetingthe relationship r₁ >r₂, where r₁ denotes the curvature in eachcornerportion of the hollow of the drawing mold, and r₂ represents thecurvature in each corner portion of the recess of the curling mold.

The method of the present invention described above produces prominenteffects as pointed out below:

1. The method of the present invention is used a curling mold having arectangular hollow in the central portion, a rectangular recess formedin the bottom portion to communicate with the hollow and sized largerthan the hollow, and a tapered portion formed in the inner surface inthe lowerend portion of the recess, the tapered portion having an angleof 0.5° to 10°. The curling mold of the particular constructionis usedin the curling step, making it possible to curl in a desired shape theopen upper end portion of the metal case having the diameter diminishedby the drawing mold. What should also be noted is that a tapered portionis formed in the inner surface in the lower end portion ofthe recessincluded in the curling mold, as described previously. When the curlingmold is engaged with the open upper end portion of the metal case,thepresence of the tapered portion permits occurring the space betweenthecircumferential region below the open end portion of the metal caseand thelower end portion of the recess included in the curling mold,with the result that the curling mold can be detached easily from themetal case after the curling process. It follows that it is possible toprevent the manufactured secondary cell from being lowered in theair-tightness because the sealing plate is prevented from falling downand the insulating gasket is prevented from being split.

2. The method of the present invention is used a drawing mold and acurlingmold meeting the relationship r₁ >r₂, where r₁ denotes thecurvature in each corner portion of the hollow of the drawing mold, andr₂ represents the curvature in each corner portion of the recess of thecurling mold. Since the curvature r₁ in each corner portion of thehollow of-the drawing mold is set larger than the curvature r₂ ineachcorner portion of the recess of the curling mold, the curvature in eachcorner portion in the open end portion of the metal case after thediameter reduction is made equal to the curvature r₁ noted above. Itfollows that, when the curling mold is engaged with the open end portionof the metal case for the curling treatment, it is possible to suppressthe force acting on the corner portions in the open end portion of themetal case. As a result, it is possible to prevent the metal platinglayerfrom peeling off the metal case and to prevent the metal case frombeing damaged, and also the curling mold can be detached easily from themetal case. Therefore, the air-tightness and reliability of themanufactured secondary cell can be enhanced.

The present invention also provides a method of manufacturing arectangularnickel-metal hydride secondary cell, comprising the steps of:

housing an electrode group and an alkali electrolyte in a rectangularcylindrical metal case having a bottom and a stepped portion along theinner surface of the metal case formed by enlarging the open end portionof the metal case, the electrode group being constructed such that anickel positive electrode and a hydrogen absorption alloy negativeelectrode are superposed one upon the other with a separator interposedbetween the adjacent positive and negative electrodes;

mounting a rectangular cylindrical insulating gasket having a bottom anda rectangular sealing plate housed therein in advance on the steppedportionof the metal case, a rectangular hole being formed in the bottomof the insulating gasket, and the sealing plate having longer sidesurfaces whichare curved outward;

reducing the diameter of the open end portion of the metal case to avalue substantially equal to that of the main body portion of the metalcase by using a drawing mold so as to press inwards the rising wall ofthe insulating gasket, the mold having a rectangular hollow in thecentral portion and a tapered portion formed by outwardly enlarging theinner surface of the hollow in the lower portion of the mold; and

abutting a curling mold against the open end portion of the metal case,so as to inwardly fold the open end portion of the metal case to form afolded portion and, thus, to permit the sealing plate to be fixed underpressure to the metal case with the insulating gasket interposedtherebetween, the curling mold having a rectangular hollow in thecentral portion, a rectangular recess formed in the bottom portion tocommunicate with the hollow and sized larger than the hollow, and atapered portion formed in the inner surface in the lower end portion ofthe recess;

wherein the hollow of the drawing mold has outwardly curved innersurfaces in the regions corresponding to the curved side surfaces of thesealing plate, and the recess included in the curling mold has outwardlycurved inner surfaces in the regions corresponding to the curved sidesurfaces ofthe sealing plate.

In the method of the present invention described above, it is desirableto have the hollow of the drawing mold shaped such that the width in thecentral portion defined by the outwardly curved inner surfaces isgreater by at least 0.5% than that in the end portion. It is alsodesirable to have the recess included in the curling mold shaped suchthat the width inthe central portion defined by the curved innersurfaces is greater by at least 0.5% than that in the end portion. Whereeach of the drawing mold and the curling mold used in the method of thepresent invention is shapedas pointed out above, it is possible toobtain a secondary cell exhibiting an improved air-tightness.

The particular method of the present invention described above permitsproducing a prominent effect. To reiterate, the diameter in the open endportion of the metal case is diminished in the present invention byusing a drawing mold having a hollow shaped in a particular fashion.Specifically, the longer inner surfaces of the drawing mold defining thehollow, which corresponds to outwardly curved side surfaces of thesealingplate, are bent to form a convex configuration. Further, the openend portion of the metal case is inwardly folded by using a curling moldhaving a rectangular recess shaped in a particular fashion.Specifically, the longer inner surfaces of the curling mold defining therectangular recess, which corresponds to the outwardly curved sidesurfaces of the sealing plate, are bent to form a convex configuration.Use of the particular curling mold makes it possible to fold inwardlythe open upper end portion of the metal case to conform with theoutwardly curved side surfaces of the sealing plate. It follows that itis possible to prevent the insulating gasket interposed between thefolded portion of the metal case and curved side surfaces of the sealingplate from being compressed excessively, making it possible to preventthe sealing plate from being deformed. Since the sealing plate isprevented from being deformed, it is possible to prevent the open endportion of the metal case, particularly the longer side of the open endportion, from being bent by the gas pressure generated within the metalcase in the event of, for example, short-circuiting or over-charging. Inother words, the secondary cell of the present invention permitsmaintaining a high air-tightness even in theevent of increase in theinternal pressure.

Further, the present invention provides a method of manufacturing arectangular nickel-metal hydride secondary cell, comprising the stepsof:

applying at least one cycle of charging-discharging cycles to arectangularnickel-metal hydride secondary cell including an electrodegroup and an alkali electrolyte, the electrode group being constructedsuch that a nickel positive electrode and a hydrogen absorption alloynegative electrode are superposed one upon the other with a separatorinterposed between the adjacent positive and negative electrodes;

permitting the secondary cell with a small residual capacity to standfor aging; and

fully charging the secondary cell after the aging step, followed bydischarging the secondary cell so as to perform a capacity screeninginspection of the secondary cell.

The aging treatment included in the method of the present inventiondescribed above is intended to activate sufficiently the hydrogenabsorption alloy negative electrode. It is desirable to carry out theaging treatment at a temperature falling with a range of between 25° C.and 60° C. If the temperature for the aging treatmentis lower than 25°C., it is difficult in some cases to activate the negative electrode. Onthe other hand, if the aging treatment is carried out at temperaturesexceeding 60° C., the electrode group housed inthe secondary cell tendsto be deteriorated.

In the method described above, the expression "small residual capacity"noted above represents the capacity obtained by discharging a secondarycell with a discharge rate of at least 0.2 C until the cell voltagereaches a predetermined voltage, so as to arrive at a level not higherthan 20% of the total discharge capacity.

The method of the present invention described above produces a prominenteffect. To reiterate, at least one cycle of charging-discharging cyclesisapplied in the method of the present invention to a rectangularnickel-metal hydride secondary cell including a positive electrodecontaining nickel hydroxide as an active substance, a negative electrodecontaining a hydrogen absorption alloy as an active substance, and analkali electrolyte, followed by permitting the secondary cell with asmallresidual capacity to stand for aging. The particular treatmentmakes it possible to sufficiently activate the negative electrode and,at the same time, to increase the capacity of the negative electrode. Asa result, it is possible to improve sufficiently the discharge voltageand the discharge capacity. This makes it possible to determineappropriately the quality of the cell itself in the step of screeningthe capacity after theaging treatment regardless of the degree ofactivation of the negative electrode included in the cell. It followsthat the number of unsatisfactory cells is decreased, leading to animprovement in the yield.What should also be noted that, where a pairedcell is prepared by using the secondary cells screened by the test notedabove, the resultant pairedcells are rendered low in variation of thedischarge capacity, which is derived from the individual secondarycells.

Some Examples of the present invention, is described below.

EXAMPLE 1

A rectangular nickel-metal hydride secondary cell in this example isconstructed as shown in FIGS. 1 to 5.

As shown in FIGS. 1 and 2, a metal case 1, which also acts as a negativeelectrode, is in the form of a cylinder having a bottom and shapedrectangular in cross section. The metal case 1 has a rectangular openupper end 2. The peripheral portion of the open end 2 of the metal case1 is folded inward to form a rectangular folded portion 3.

As shown in FIGS. 3 and 4, the folded portion 3 has an angle of 88° atthe corner portion and an angle of 63° at the side interposed betweentwo adjacent corner portions. An inwardly projecting stepped portion 4is formed below the folded portion 3 of the metal case 1. An electrodegroup 5 is constructed such that a hydrogen absorption alloy negativeelectrode 6 and a positive electrode 8 containing nickel hydroxide as anactive substance wrapped in a bag-like separator 7 are alternatelysuperposed one upon the other. The electrode group 5 is housedin themetal case 1 such that the negative electrode 6 is brought into contactwith the inner surface of the metal case 1.

Also contained in the metal case 1 is an alkali electrolyte formed of amixed solution consisting of 7N-KOH and 1N-LiOH. A sealing lid group 9,which also performs the explosion-preventing function and acts as aterminal of the positive electrode, is positioned in a region surroundedwithin the metal case 1 by the folded portion 3 and the stepped portion4.The sealing lid group 9 comprises a rectangular sealing plate 11having a gas releasing hole 10 formed in the central portion, an elasticvalve body12 formed of, for example, a synthetic rubber, and ahat-shaped terminal cap 13 having a gas releasing hole (not shown)formed therein. The elasticvalve body 12 is disposed on the sealingplate 11 to close the gas releasing hole 10. The terminal cap 13 iswelded to the sealing plate 11 in a manner to surround the elastic valvebody 12.

An insulating gasket 14, which is in the form of a cylinder having abottom, shaped rectangular in its cross section, and having arectangular hole formed in the bottom, is interposed between the innersurface near the open end of the metal case 1 and the side surface ofthe sealing plate11. The sealing plate 11 of the sealing lid group 9 ishermetically fixed to the metal case 1. The insulating gasket 14 isformed of, for example, nylon 6, 6 containing carbon black.

A positive electrode lead 15 is connected at one end to the nickelpositiveelectrode 8 and, at the other end, to the lower surface of thesealing plate 11. An insulating plate 16 having a rectangular hole 17formed in the central portion is disposed to extend over a flangeportion 19 of the terminal cap 13 and the folded portion 3 of the metalcase 1 such that a projecting portion 18 of the terminal cap 3 isengaged with the rectangular hole 17 of the insulating plate 16, asshown in FIG. 5. An outer tube 20, which is formed of a heat-shrinkableresin, is used to cover the outer surface in the main body portion ofthe metal case 1, the outer peripheral portion in the bottom of themetal case and the peripheral portion of the insulating plate 16, withthe result that the insulating plate 16 is fixed to the folded portion 3of the metal case 1 and the flange portion 19 of the terminal cap 13.

Where a gas is generated within the metal case 1 by, for example,over-charging or short-circuiting in the secondary cell constructed asshown in FIGS. 1 to 5, the gas pressure is applied to the elastic valvebody 12 made of an elastic material via the gas releasing hole 10 formedin the sealing plate 11. In this case, the elastic valve body 12 iselastically deformed to provide a clearance between the elastic valvebody12 and the sealing plate 11. It follows that the gas generatedwithin the metal case 1 is released to the outside through the clearancenoted above and the gas releasing hole (not shown) formed in theterminal cap 13, withthe result that the secondary cell is preventedfrom being broken.

To reiterate, the secondary cell of the construction described abovecomprises the folded portion 3 formed by folding the open end portion 2ofthe metal case 1. It should be noted that the folding angle of thefolded portion 3 in the corner portion is set at 88°, as shown in FIG.3. On the other hand, the folding angle of the folded portion 3 in theside interposed between two adjacent corner portions is set at 63°,which is smaller than the folding angle in the corner portion, as showninFIG. 4. The particular folding angles defined in the present inventionmakeit possible to maintain a high air-tightness of the cell and toprevent themetal case 1 from bearing deflection or strain without fail.

In Example 1 described above, it is desirable for the folding length ofthefolded portion 3 in the corner portion of the open end 2 of the metalcase 1 to be greater than the thickness of the metal case 1. It is alsodesirable to determine the curvature radius Rc at the corner portion inthe folded portion 3 of the metal case 1 to meet formula (1) givenbelow:

    1.5t≦Rc≦4t                                   (1)

where "t" denotes the thickness of metal case 1.

The particular construction meeting the requirements described abovemakes it possible to prevent the metal case 1 from being deformed whenthe folded portion 3 is formed and to improve the air-tightness of thesecondary cell.

In Example 1, it is also desirable to for the curvature radius Rb in thecorner portion of the bottom of the metal case 1 to meet formula (2)givenbelow:

    Rb≦Rc                                               (2)

In other words, the curvature radius Rb noted above should not be largerthan the curvature radius Rc at the corner portion in the folded portion3of the metal case 1.

Where all the requirements (1) and (2) given above are satisfiedtogether, the metal case 1 is allowed to exhibit an increased mechanicalstrength atthe bottom portion, and at the same time, the metal case 1 isprevented from being deformed in the step of folding the open endportion 2 of the metal case 1 without fail. The values of t, Rc and Rbnoted above are desirably set at, for example, 0.4 mm, 1 mm and 0.8 mm,respectively.

In the present invention, it is also preferable to determineappropriately the amount of the alkali electrolyte contained in themetal case 1. To be more specific, the amount of the electrolyte ispreferably 90% by volume or less based on the effective free space ofthe metal case 1, which is determined by subtracting the effectivevolumes excluding the porosities of the nickel positive electrode 8, thehydrogen absorption alloy negativeelectrode 6 and the separator 7 fromthe volume occupied by the electrode group 5, i.e., A×B, where A denotesthe length of the electrode group 5 and B represents the bottom area ofthe metal case 1. It is also preferable for the amount of the alkalielectrolyte to be at least 1.7 ml/Ah of the cell capacity. Where theamount of the electrolyte contained in the metal case 1 is determined tomeet these conditions, the oxygen gasgenerated from the positiveelectrode 8 in the charging step can be promptly absorbed by the surfaceof the negative electrode 6, as seen fromsamples A to D shown in Table 1given below. It follows that it is possibleto suppress the increase inthe internal pressure of the secondary cell andto ensure satisfactorycharging-discharging characteristics.

                  TABLE 1                                                         ______________________________________                                                       A     B       C       D                                        ______________________________________                                        Electrolyte amount (vol %)                                                                     80      85      90    95                                     based on effective                                                            free space                                                                    Electrolyte amount (mL)/Ah                                                                     1.7     1.85    2.00  2.10                                   of cell capacity                                                              Evaluation                                                                    Max. cell inner pressure                                                                       2.1     3.5     5.2   13                                     at 150% charging step                                                         at 0.5 C                                                                      The number of charge-                                                                          350     440     500   85                                     discharge cycles reaching                                                     60% of initial discharge                                                      capacity                                                                      ______________________________________                                    

As described previously, the insulating gasket 14 included in thesecondarycell of Example 1 is formed of nylon 6, 6 containing carbonblack. What should be noted is that the carbon black provides crystalnuclei in the step of preparing the insulating gasket 14, leading to animproved crystallinity of the gasket 14. It follows that the insulatinggasket 14 is allowed to exhibit improvements in compression repulsivity(compressioncreep characteristics), weatherability and oxidationresistance, with the result that the air-tightness of the secondary cellcan be further improved.

A comparative experiment was conducted in an attempt to look into theeffect produced by the carbon black addition to nylon 6, 6 inpreparation of the insulating gasket. Specifically, prepared was asecondary cell A provided with an insulating gasket formed of nylon 6, 6containing 0.2% byweight of carbon black. Also prepared was a secondarycell B provided with an insulating gasket formed of nylon 6, 6 alone.These two secondary cellsA and B were charged at 0.2 C under atemperature cycle ranging between -10° C. and +65° C. Many samples ofthe charged secondary cells A and B were left to stand, followed bycounting with Mil-Std-202F the number of cells in which the electrolyteleakage was found so as to obtain a leakage occurrence rate. FIG. 6shows the results. As seen from the experimental data shown in FIG. 6,the secondary cell A provided with an insulating gasket formed of nylon6, 6 containing carbon black was found to markedly suppress the leakageoccurrence rate, compared with the secondary cell B provided with aninsulating gasket formed of nylon 6, 6 alone. In other words, theinsulating gasket formed of nylon 6, 6 containing carbon black permitsmarkedly improving the air-tightness of the secondary cell.

In the secondary cell in Example 1, it is desirable to set thecompression ratio applied to that portion of the insulating gasket 14which is interposed between the side of the open end portion 2 of themetal case 1 and the sealing plate 11 at 15 to 40%. In addition, thecompression ratio noted above is desirably higher than the compressionratio applied to thatportion of the insulating gasket 14 which isinterposed between the corner portion in the open end portion 2 of themetal case 1 and the sealing plate 11.

The compression ratio noted above is determined by formula (3) givenbelow:

    Compression ratio (%)=(tb-ta)/tb×100                 (3)

where tb denotes the thickness of the rising portion of the insulatinggasket 14 before the compression, and ta represents the thickness of therising portion of the insulating gasket 14 after the compression.

Where the insulating gasket 14 is compressed under the conditionspecified above, it is possible to enable the insulating gasket 14 toretain a repulsive elastic force large enough to overcome the bendingforce when the side portion in the vicinity of the open end portion 2 ofthe metal case 1 is expanded outward by the increase in the internalpressure of thesecondary cell. It follows that the air-tightness of thesecondary cell canbe further improved.

In the secondary cell in Example 1, it is desirable to form arectangular frame-shaped eaves portion 14b for provisionally fixing thesealing plate 11 to the inner surface of the rising portion 14a of theinsulating gasket14, as shown in FIG. 7. Where the insulating gasket isprovided with such an eaves portion, it is desirable not to form theeaves portion in the corner portion or to make a projecting length ofthe eaves portion positioned in the corner portion shorter than that ofthe eaves portion positioned in the side portion interposed between twoadjacent corner portions.

To be more specific, where an insulating gasket having an eaves portionformed at the corner portion is formed by an injection molding, followedby detaching the mold from the molded insulating gasket, the eavesportionpositioned in the corner portion is not as likely as to beexpanded outwardthan the eaves positioned in the side portion. Itfollows that it is difficult in some cases to detach the mold from theeaves portion positioned in the corner portion. It should also be notedthat, where the sealing plate is inserted into the insulating gaskethaving an eaves portion positioned in the corner portion, it isnecessary to allow the sealing plate to push the eaves portionpositioned in the corner portion because the eaves portion positioned inthe corner portion is not as likely as to be expanded outward than theeaves positioned in the side potion. As a result, the eaves portionpositioned in the corner portion isscratched off by the sealing plateand, thus, the sealing plate is disposedwithin the insulating gasket,with the eaves portion, which is scratched off by the sealing plate,held between the sealing plate and the insulating gasket. It followsthat the bonding strength between the insulating gasket and the sealingplate is lowered, leading to reduction in the air-tightness of thesecondary cell.

In the insulating gasket of the particular shape described above, it isdesirable to form an eaves portion for provisionally fixing the longerside of the sealing plate without forming an eaves portion forprovisionally fixing the shorter side of the sealing plate.Alternatively,it is desirable to make the a projecting length of eavesportion for provisionally fixing the shorter side of the sealing plateshorter than that of the eaves portion for provisionally fixing thelonger side of the sealing plate. The insulating gasket of theparticular construction permits further facilitating the release of themold and also permits facilitating the insertion of the sealing plateinto the insulating gasket.

The mold release capability and the insertion capability of a sealingplateinto an insulating gasket were examined in respect of insulatinggasket samples A, B and C provided with eaves portions sized as shown inTable 2.As seen from Table 2, an eaves portion for provisionally fixingthe corner portion of the sealing plate was not formed in each ofsamples A and B. Onthe other hand, sample C was provided with eavesportions for provisionallysupporting the corner portion, the shorterside, and the longer side of thesealing plate. The results of theexamination are also shown in Table 2.

                  TABLE 2                                                         ______________________________________                                                       A     B         C                                              ______________________________________                                        Projecting length of                                                          eaves portion of                                                              insulating gasket                                                             (mm)                                                                          Longer side      0.07    0.1       0.05                                       Shorter side     0.05    0         0.05                                       Corner           0       0         0.05                                       Evaluation                                                                    Mold Release Capability                                                                        Good    Good      Somewhat                                                                      Good                                       Insertion Capability                                                                           Good    Good      Somewhat                                                                      Good                                       ______________________________________                                    

Table 2 shows that, in the insulating gasket sample A, an eaves portionwasnot formed in the corner portion, and a projecting length of theeaves portion for provisionally fixing the shorter side of the sealingplate wassmaller than that of the eaves portion for provisionally fixingthe longer side of the sealing portion. On the other hand, theinsulating gasket sample B was provided with only an eaves portion forprovisionally fixing the longer side of the sealing plate alone. Asshown in Table 2, each of these insulating gasket samples A and B wasfound to be satisfactory in each of the mold release capability and theinsertion capability of the sealing plate into the insulating gasket.Naturally, the secondary cell provided with an insulating gasketconstructed as in any of the insulatinggasket samples A and B is enabledto exhibit an improved air-tightness.

It is desirable to prepare the insulating gasket 14 included in thesecondary cell in Example 1 by means of injection molding using a moldprovided with at least two injection gates. Where the injection moldingmold used for preparing the insulating gasket 14 is provided with twoinjection gates positioned in symmetry with respect to the center of themolding, i.e., the insulating gasket to be obtained, the resultantinsulating gasket 14 is rendered uniform in the resin density over theentire region, compared with the case where the resin is injected intothemold cavity through only one injection gate for preparation of theinsulating gasket. To be more specific, a mold provided with twoinjectiongates permits suppressing the weld line denoting a differencein the resin density of the resultant insulating gasket 14, with theresult that the insulating gasket 14 is prevented from being cracked.

Further, it is desirable to inject the resin material into the moldcavity through a gate positioned in the bottom portion of the injectionmolding mold in preparation of the insulating gasket 14. It should benoted that the resin in that portion of the insulating gasket 14 whichcorresponds tothe gate region noted above differs in density andcrystallinity from the other portion, with the result that cracking islikely to take place in the insulating gasket 14 in the step of sealingthe metal case 1. In orderto overcome the difficulty, it is desirable toinject the resin material through a gate positioned in the bottomportion of the mold in preparing the insulating gasket 14. What shouldbe noted is that, in this case, force is not exerted in the step ofsealing the metal case 1 on that portion of the resultant insulatinggasket 14 which corresponds to the gate region positioned in the bottomof the injection molding mold. In other words, the particular moldingtechnique described above permits improving the mechanical strength inthe rising portion, to which the greatest force is exerted in thesealing step, of the insulating gasket 14. It follows that it ispossible to further improve the air-tightness ofthe secondary cell.

The terminal cap 13 included in the secondary cell of Example 1comprises the flange portion 19 as described previously. It is desirableto form a plurality of projections for resistance welding on the lowersurface of the flange portion 19. These projections is preferablypositioned in symmetry with respect to the center of the terminal cap13. To be more specific, it is desirable to form, for example, two orfour projections inthe particular fashion. More preferably, it isdesirable to form four projections 21 in the four corner portions in thelower surface of the flange portion 19 of the terminal cap 13, as shownin FIGS. 8 and 9. As apparent from the drawings, these four projections21 are positioned in symmetry with respect to the center of the terminalcap 13. The terminal cap 13 constructed as shown in FIGS. 8 and 9permits increasing the mounting strength of the terminal cap 13 againstthe sealing plate 11 and also permits decreasing the nonuniformity inthe size of the fused portionof the projection 21 in the welding stepagainst the sealing plate 11.

As shown in FIG. 9, it is desirable for the projecting amount td of theprojection 21 to be 0.5 to 2 times as much as the thickness of theterminal cap 13, and for the curvature radius Rd of the projection 21 tobe 1.5 to 2.4 times as much as the thickness of the terminal cap 13.

It is desirable for the elastic valve body 12 included in the secondarycell in Example 1 to be formed of a material having a tensile strengthof at least 100 kg/cm² and a tensile strength deterioration rate nothigher than 2% in the ozone resistance deterioration test in which thesample is left to stand for 72 hours under an atmosphere of 40° C.having an ozone concentration of 50 ppm. The material meeting theparticular requirements includes, for example, anethylene-propylene-nonconjugated diene compound terpolymer (EPDM) and anethylene-propylene copolymer.

As described previously, the sealing plate 11 included in the secondarycell in Example 1 is provided with the gas releasing hole 10. As shownin FIG. 10, it is desirable for the curvature radius Re in the cornerportionof the upper surface of the sealing plate 11 defining the gasreleasing hole 10 to be at least 0.05 mm. In this case, the cornerportion of the gas releasing hole 10 is rendered moderate, with theresult that the use of the sealing plate 11 provided with the particulargas releasing hole 10makes it possible to prevent the elastic valve body12 from being broken when the elastic valve body 12 is deteriorated. Tobe more specific, the elastic valve body 12 is deteriorated when theelastic valve body 12 is moved up and down within the free space definedbetween the terminal cap 13 and the sealing plate 11 by the pressure ofthe gas generated by the over-charging state over a long period of time.It follows that, if the curvature radius Re of the gas releasing hole 10is smaller than 0.05 mm, the corner portion of the gas releasing hole 10is rendered too sharp to prevent the deteriorated elastic valve bodyfrom being broken by the corner portion in question.

It is desirable to form the projection 18 of the terminal cap 13 in ashapesubstantially conforming with the rectangular hole 17 formed in theinsulating plate 16 as shown in FIG. 5. Where the projection 18 of theterminal cap 13 is shaped in this particular fashion, it is possible toguide the insulating plate 16 naturally into an appropriate positionwhen the insulating plate 16 is engaged with the projection 18 of theterminal cap 13. It should also be noted that the insulating plate 16 isfixed to the upper surface of the terminal cap 13. It follows that theprojection 18 of the particular shape permits suppressing inconveniencessuch as detachment of the insulating plate 16 from the terminal cap 13when the insulating plate 16 is rotated, leading to an improvedreliability and a high productivity of the secondary cell.

EXAMPLE 2

FIGS. 11A to 11I collectively show the drawing mold and the curling moldused in Example 2. As shown in FIGS. 11A and 11B, a metal case, which isdescribed later, is disposed on a lower mold 22. A first drawing mold 25movable in a vertical direction is arranged above the lower mold 22. Thefirst drawing mold 25 comprises a hollow 23, which is rectangular incrosssection, and a tapered portion 24 formed by outwardly expanding by5° the inner surface in the lower portion of the hollow 23. A knock-out27 having a rectangular frame-like projection 26 formed in the lowerportion thereof is vertically movable within the hollow 23. Thecurvature radius r₁ in the corner portion of the hollow 23 of the firstdrawing mold 25, which is shown in FIG. 12, is 0.81 mm in Example 2.

As shown in FIGS. 11C and 11D, a first curling mold 30 is arranged abovethe lower mold 22. The first curling mold 30 comprises a hollow 28 whichis rectangular in cross section, and a rectangular recess 29 formed inthebottom portion in a manner to communicate with the hollow 28 andsized larger than the hollow 28. A knock-out 32 having a rectangularframe-like projection 31 formed in the periphery of the bottom portionis vertically movable within the hollow 28 and the recess 29. Eachcorner portion of therecess 29 has a curvature radius r₂, which is shownin FIG. 13, of 0.79 mm. As shown in FIG. 14, a tapered portion 33 havinga tapering angleof 2° is formed in the inner surface in the lower endportion of therecess 29.

As shown in FIGS. 11E and 11F, a second drawing mold 36, which ismovable in a vertical direction, is arranged above the lower mold 22.The second drawing mold 36 comprises a central hollow 34, which isrectangular in cross section, and a tapered portion 35 formed byoutwardly expanding by 4° the lower end portion of the hollow 34. Aknock-out 38 having a rectangular frame-like projection 37 formed in theperiphery of the lower end portion is vertically movable within thehollow 34. It should be notedthat each corner of the hollow 34 has acurvature radius of 0.81 mm.

As shown in FIGS. 11G and 11H, a second curling mold 41 is arrangedabove the lower mold 22. The second curling mold 41 comprises a hollow39 which is rectangular in cross section and a rectangular recess 40formed in the bottom in a manner to communicate with the hollow 39 andsize larger than the hollow 39. A knock-out 43 having a rectangularframe-like projection 42 formed in the periphery of the lower portion isvertically movable within the hollow 39 and the recess 40. A taperedportion 44 was formed byoutwardly expanding by 2° the lower end portionof the recess 40. Further, each corner of the recess 40 has a curvatureradius of 0.79 mm.

Let us describe in detail a method of the present invention formanufacturing a rectangular nickel-metal hydride secondary cell.

As shown in FIG. 11A, in the first step, a stepped portion 46 is formedin the upper portion of a metal case 45, which is cylindrical andrectangularin cross section and a has wall thickness of 0.4 mm, byoutwardly expandingthe upper portion of the metal case 45. The metalcase 45 has a longer sideL₁ of 13.8 mm and a shorter side of 6.8 mm.After the expansion, a rising portion 47 having a longer side L₂ of 14.2mm and a shorter side of 7.2 mm is formed upward of the stepped portion46. After the expanding step, an electrode group 51 prepared byalternately laminating apositive electrode 49 and a negative electrode50 is disposed in the metal case 45. The positive electrode 49 iscovered with a bag-like separator 48and contains nickel hydroxide as anactive substance. On the other hand, the negative electrode 50 containsa hydrogen absorption alloy as an active substance. Then, an alkalielectrolyte is contained in the metal case 45.

Then, a sealing lid group 54 performing a function of preventingexplosion and acting as a terminal is mounted on an insulating gasket 53which is inthe form of a rectangular cylinder having a bottom and arectangular hole 52 formed in the bottom portion. The insulating gasket53 having the sealing lid group 54 mounted thereon is put on the steppedportion 46 positioned below the rising portion 47 of the metal case 45.The sealing lid group 54 comprises a rectangular sealing plate 56 havinga gas releasing hole 55 formed in the center, an elastic valve body 57formed of, for example, a synthetic rubber, and a hat-shaped terminalcap 58 having a gas releasing hole (not shown) formed therein. Theelastic valve body 57 is disposed to cover the gas releasing hole 55 ofthe sealing plate 56. The terminal cap 58 is arranged to surround theelastic valve body 57 and is welded to the sealing plate 56. Further, apositive electrode lead 59 connected at one end to the positiveelectrode 49 and, at the other end, to the lower surface of the sealingplate 56. Under thiscondition, the metal case 45 is disposed on thelower mold 22, and the first drawing mold 25 is arranged above the metalcase 45.

In the next step, the first drawing mold 25 is moved downward, whileallowing the projection 26 of the knock-out 27 to hold down the open endportion of the metal case 45, such that the open upper end portion ofthe metal case 45 is positioned within the hollow 23 of the firstdrawing mold25, as shown in FIG. 11B. In this step, the open upper endportion of the metal case 45 is diminished to have the width L3 equal tothat in the insulating gasket 53. At the same time, the stepped portion46 below the open upper end portion of the metal case 45 is movedinward. Then, the first drawing mold 25 is moved upward, while allowingthe projection 26 ofthe knock-out 27 to hold downward the open endportion of the metal case 45, so as to detach the first drawing mold 25from the metal case 45. Further, the first curling mold 30 having theinner surface of the recess 29 coated with a lubricant is arranged abovethe metal case 45 after the first drawing mold 25 was removed, as shownin FIG. 11C.

In the next step, the first curling mold 30 is moved downward to allowthe projection 31 of the knock-out 32 to abut against the sealing plate56 as shown in FIG. 11D. In this step, the open end portion of the metalcase 45is allowed to abut against the inner surface of the recess 29 ofthe first curling mold 30 so as to fold inwardly both the open end ofthe metal case45 and the rising portion of the insulating gasket 53.Then, the first curling mold 30 is moved upward, while allowing theprojection 31 of the knock-out 32 to hold downward the upper surface ofthe sealing plate 56, so as to detach the first curling mold 30 from themetal case 45. Further,the second drawing mold 36 is arranged above themetal case 45 after the first curling mold 30 was removed, as shown inFIG. 11E.

As shown in FIG. 11F, the second drawing mold 36 is moved downward inthe next step, while allowing the projection 37 of the knock-out 38 tohold downward the open end portion of the metal case 45, such that theopen endportion of the metal case 45 is positioned within the hollow 34of the second drawing mold 36. In this step, the open upper end portionof the metal case 45 is diminished to have the width L₁ equal to that inthebody portion, which is not diminished, of the metal case 45. At thesame time, the stepped portion 46 of the metal case 45 is moved inwardto have a width L₄. After the drawing treatment described above, thesecond drawing mold 36 is moved upward, while allowing the projection 37of the knock-out 38 to hold downward the open end portion of the metalcase 45, so as to detach the second drawing mold 36 from the metal case45, followed by arranging the second curling mold 41 above the metalcase 45, as shown in FIG. 11G.

The second curling mold 41 is then moved downward, as shown in FIG. 11H.Inthis step, the inner surface of the recess 40 of the second curlingmold 41is allowed to abut against the open end portion of the metal case45 so as to curl the open end portion of the metal case 45 and tocompress the rising portion of the insulating gasket 53. Finally, thesecond curling mold 41 is moved upward, while allowing the projection 42of the knock-out43 to hold downward the sealing plate 56 so as to detachthe second curlingmold 41 and the lower mold 22 from the metal case 45as shown in FIG. 11I and, thus, to finish the sealing process.

In the manufacturing method described above, used is the first curlingmold30 having the hollow 28 formed in the central portion, the hollow 28being rectangular in cross section, and the rectangular recess 29 formedin the bottom portion to communicate with the hollow 28 and sized largerthan thehollow 28, as shown in FIG. 11D and FIG. 13. What should benoted is that the tapered portion 33 is formed in the inner surface inthe lower end portion of the rectangular recess 29, making it possibleto fold the open end portion of the metal case 45, whose diameter hasbeen diminished, in adesired shape. The tapered portion 33 also servesto occur the space between the periphery in the open end portion of themetal case 45 and thelower end portion of the recess 29, with the resultthat, in the step of moving the first curling mold 30 upward so as todetach the first curling mold 30 from the metal case 45, it is possibleto decrease the friction between the periphery in the open end portionof the metal case 45 and theinner surface of the recess 29. It followsthat the first curling mold 30 can be detached quite easily from themetal case 45, making it possible toobtain a secondary cell with amarkedly improved air-tightness. Incidentally, the second curling mold41 also produces the effect similar to that produced by the firstcurling mold 30.

What should also be noted is that, in the manufacturing method describedabove, the corner portion of the hollow 23 in the first drawing mold 25has a curvature radius r₁ of 0.81 mm. On the other hand, the cornerportion of the recess 29 in the first curling mold 30 has a curvatureradius r₂ of 0.79 mm. In other words, the first drawing mold 25 and thefirst curling mold 30 used in the method described above meet therequirement of r₁ >r₂ specified in the present invention, with theresult that the open end portion of the metal case 45 can be diminishedthe diameter so as to impart a curvature radius equal to that of thecorner of the hollow 23 in the first drawing mold 25 to the corner inthe open end portion of the metal case 45. It follows that it ispossible to diminish the force exerted on the corner when the open endportion of the metal case 45 is folded inward, making it possible toprevent the metal case 45 from being deformed in the folding step. Whatshould also be noted is that the first curling mold 30 is prevented frombiting the corner of the open end portion of the metal case 45, with theresult that the first curling mold 30 can be detached more easily fromthemetal case 45. It follows that it is possible to prevent thereduction in the air-tightness, said reduction being derived from thewarping of the sealing plate 56 and from the cracking of the insulatinggasket 53. Incidentally, the second drawing mold 36 and the secondcurling mold 41 also produce the effects similar to those produced bythe first drawing mold 25 and the first curling mold 30.

As a matter of fact, the secondary cell manufactured in Example 2 wasfoundto exhibit excellent properties, when compared with ComparativeExample 1 described below.

COMPARATIVE EXAMPLE 1

A secondary cell was manufactured as in Example 2, except that a taperedportion was not formed in the inner surface in the lower end portion ofthe recess included in the curling mold. In Comparative Example 1,however, the curling mold was moved upward after the curling process,while allowing the knock-out to hold strongly downward the sealingplate, so as to detach the curling mold from the metal case. As aresult, warpingof the sealing plate and the dropping of the insulatinggasket were found in the manufactured secondary cell.

In each of Example 2 and Comparative Example 1, the holding forceexerted by the knock-out on the sealing plate was measured in the stepof detaching the curling mold from the metal case after the curlingprocess. The holding force was found to be as small as only 35 kg inExample 2 in contrast to 70 kg for Comparative Example 1.

It should be noted that it is desirable to coat that surface of each ofthecurling mold and the drawing mold which is brought into contact withthe surface of the metal case with a lubricant so as to lower thefriction in the contact region.

EXAMPLE 3

The drawing mold and the curling mold used in Example 3 are shown inFIGS. 15A to 15I.

As shown in FIGS. 15A and 15B, a metal case, which is described later,is mounted on a lower mold 60. On the other hand, a first drawing mold63, which is movable in a vertical direction, is arranged above thelower mold60. The first drawing mold 63 has a hollow 61 formed in thecentral portion. Also, the inner surface in the lower portion of thehollow 61 is expanded outward so as to form a tapered portion 62. Aknock-out 65 havinga rectangular frame-like projection 64 formed in theperiphery of the lowerportion is movable in a vertical direction withinthe hollow 61 of the first drawing mold 63. As shown in FIG. 16, thehollow 61 is barrel-shapedin its cross section such that the innersurfaces 66 of the first drawing mold 63 defining longer sides of thehollow 61, the longer sides corresponding to outwardly curved sidesurfaces of a sealing plate which is described later, are curved in aconvex configuration. To be more specific, the hollow 61 is sized at16.62 mm in its longer side L₅, at 5.87 mm in its width L₆ in thecentral portion of the curved region, and at 5.72 mm in its shorter sideL₇, or the end of the curved region.

As shown in FIGS. 15C and 15D, a first curling mold 69 movable in avertical direction is arranged above the lower mold 60. The firstcurling mold 69 comprises a hollow 67, which has a rectangular crosssection and is formed in the central portion of the first curling mold69, and a rectangular recess 68 communicating with the hollow 67 andsized larger than the hollow 67. A knock-out 71 having a rectangularframe-like projection 70 formed in the periphery of the lower portion ismovable in avertical direction within the hollow 67 and the recess 68.As shown in FIG.17, the recess 68 is barrel-shaped in its cross sectionsuch that the innersurfaces 72 of the first curling mold 69 defininglonger sides of the recess 68, the longer sides corresponding tooutwardly curved side surfaces of a sealing plate which is describedlater, are curved in a convex configuration. To be more specific, therecess 68 is sized at 16.62mm in its longer side L₈, at 5.87 mm in itswidth L₉ in the central portion of the curved region, and at 5.72 mm inits shorter side L₁₀, or the end of the curved region.

As shown in FIGS. 15E and 15F, a second drawing mold 75 which is movableina vertical direction is arranged above the lower mold 60. The seconddrawing mold 75 comprises a hollow 73, which is rectangular in its crosssection and formed in the central portion of the second drawing mold 75.The inner surface in the lower end portion of the hollow 73 is outwardlyexpanded so as to form a tapered portion 74. A knock-out 77 having arectangular frame-like projection 76 formed in the periphery of thelower portion is movable within the hollow 73. It should be noted thatthe hollow 73 is barrel-shaped in its cross section such that the innersurfaces defining longer sides of the hollow 73, the longer sidescorresponding to outwardly curved side surfaces of a sealing plate whichis described later, are curved in a convex configuration. To be morespecific, the hollow 73 is sized at 16.62 mm in its longer side, at 5.87mm in its width in the central portion of the curved region, and at 5.72mm in its shorter side, or the end of the curved region.

As shown in FIGS. 15G and 15H, a second curling mold 80 movable in avertical direction is arranged above the lower mold 60. The secondcurlingmold 80 comprises a rectangular hollow 78 and a rectangularrecess 79 formed in the bottom to communicate with the hollow 78 andsized larger than the hollow 78. A knock-out 82 having a rectangularframe-like projection 81 formed in the periphery of the lower endportion is movable in a vertical direction through the hollow 78 and therecess 79. It shouldbe noted that the recess 79 is barrel-shaped in itscross section such thatthe inner surfaces defining longer sides of therecess 79, the longer sidescorresponding to outwardly curved sidesurfaces of a sealing plate which isdescribed later, are curved in aconvex configuration. To be more specific,the recess 79 is sized at16.62 mm in its longer side, at 5.87 mm in its width in the centralportion of the curved region, and at 5.72 mm in its shorter side, or theend of the curved region.

Let us describe in detail how to manufacture a rectangular nickel-metalhydride secondary cell.

As shown in FIG. 15A, in the first step, a stepped portion 84 is formedin the upper portion of a metal case 83, which is cylindrical andrectangularin cross section and a has wall thickness of 0.4 mm, byoutwardly expandingthe upper portion of the metal case 83. The metalcase 83 has a longer sideL₁₁ of 16.4 mm and a shorter side of 5.5 mm.After the expansion, a rising portion 85 having a longer side L₁₂ of16.8 mm and a shorter side of 5.9 mm is formed upward of the steppedportion 84. After the expanding step, an electrode group 89 prepared byalternately laminating apositive electrode 87 and a negative electrode88 is disposed in the metal case 83. The positive electrode 87 iscovered with a bag-like separator 86and contains nickel hydroxide as anactive substance. On the other hand, the negative electrode 88 containsa hydrogen absorption alloy as an active substance. Then, an alkalielectrolyte is contained in the metal case 83.

Then, a sealing lid group 92 performing a function of preventingexplosion and acting as a terminal is mounted on an insulating gasket 91which is inthe form of a rectangular cylinder having a bottom and arectangular hole 90 formed in the bottom portion. The insulating gasket91 having the sealing lid group 92 mounted thereon is put on the steppedportion 84 positioned below the rising portion 85 of the metal case 83.The sealing lid group 92 comprises a rectangular sealing plate 94 havinga gas releasing hole 93 formed in the center, an elastic valve body 95formed of, for example, a synthetic rubber, and a hat-shaped terminalcap 96 having a gas releasing hole (not shown) formed therein. As shownin FIG. 18, the sealing plate 94 is shaped such that the mutually facingtwo longer sides are outwardly expanded in the central portion in aconvex configuration. To be more specific, the sealing plate 94 is sizedat 0.8 mm in wall thickness, at 4.25 mm in the width L₁₃ in the centralportion of the curved longer side surfaces, and at 4.1 mm in the end ofthe curved longer side surfaces. In order to improve the air-tightnessof the manufactured secondary cell, it is desirable to set the length ofthe longer side surfaces of the sealing plate 94 at a level not largerthan 40times as much as the thickness of the metal case 83. In thisembodiment, the the longer side surfaces of the sealing plate 94 is setat 15 mm. The elastic valve body 95 is disposed to cover the gasreleasing hole 93 of the sealing plate 94. The terminal cap 96 isarranged to surround the elastic valve body 95 and is welded to thesealing plate 94. Further, a positive electrode lead 97 connected at oneend to the positive electrode 87 and, at the other end, to the lowersurface of the sealing plate 94. Under this condition, the metal case 83is disposed on the lower mold 60, and the first drawing mold 63 isarranged above the metal case 83.

In the next step, the first drawing mold 63 is moved downward, whileallowing the projection 64 of the knock-out 65 to hold downward the openend portion of the metal case 83, such that the open upper end portionof the metal case 83 is positioned within the hollow 61 of the firstdrawing mold 63, as shown in FIG. 15B. What should be noted is thedownward movement of the first drawing mold 63 causes the insulatinggasket 91 to be somewhat compressed. At the same time, the steppedportion 84 below theopen upper end portion of the metal case 83 is movedinward. Then, the first drawing mold 63 is moved upward so as to detachthe first drawing mold 63 from the metal case 83. Further, the firstcurling mold 69 is arranged above the metal case 83 after the firstdrawing mold 63 was removed, as shown in FIG. 15C.

In the next step, the first curling mold 69 is moved downward to allowthe projection 70 of the knock-out 71 to abut against the sealing plate94 as shown in FIG. 15D. In this step, the open end portion of the metalcase 83is allowed to abut against the inner surface of the recess 68 ofthe first curling mold 69 so as to fold inwardly both the open end ofthe metal case83 and the rising portion of the insulating gasket 91.Then, the first curling mold 69 is moved upward so as to detach thefirst curling mold 69 from the metal case 83. Further, the seconddrawing mold 75 is arranged above the metal case 83 after the firstcurling mold 69 was removed, as shown in FIG. 15E.

As shown in FIG. 15F, the second drawing mold 75 is moved downward inthe next step, while allowing the projection 76 of the knock-out 77 tohold downward the open end portion of the metal case 83, such that theopen endportion of the metal case 83 is positioned within the hollow 73of the second drawing mold 75. In this step, the open upper end portionof the metal case 83 is diminished to have the width L₁₁ equal to thatin the body portion, which is not diminished, of the metal case 83. Atthe same time, the stepped portion 84 of the metal case 83 is movedinward to have a width L₁₅ of 15.4 mm. After the drawing treatmentdescribed above, the second drawing mold 69 is moved upward so as todetach the second drawing mold 75 from the metal case 83, followed byarranging the second curling mold 80 above the metal case 83 after thesecond drawing mold 75 was removed, as shown in FIG. 15G.

The second curling mold 80 is then moved downward to allow theprojection 81 of the knock-out 82 to abut against the sealing plate 94as shown in FIG. 15H. In this step, the inner surface of the recess 79of the second curling mold 80 is allowed to abut against the open endportion of the metal case 83 so as to further fold inward the open endportion of the metal case 83 and the rising portion of the insulatinggasket 91. As a result, formed is a folded portion 98 along theperiphery of the sealing plate 94 such that the folded portion 98conforms with the side surfaces of the sealing plate 94, the longer sidesurfaces being expanded outward in a convex configuration. Finally, thesecond curling mold 80 and the lower mold 60 are detached from the metalcase 83 as shown in FIG. 15I so as to obtain a rectangular nickel-metalhydride secondary cell.

In the manufacturing method described above, the diameter of the openend portion of the metal case 83 is diminished by using the firstdrawing mold63 having the inner surface 66 of the hollow 61, whichcorresponds to the curved side surfaces of the sealing plate 94,expanded outward in a convexform. As shown in FIG. 19 the open endportion of the metal case 83 can be fold inward in a manner to conformwith the curved side surfaces of the sealing plate 94 by using the firstcurling mold 69 having the inner surface 72 of the recess 60, whichcorresponds to the curved side surfacesof the sealing plate 94, expandedoutward in a convex. As a result, it is possible to prevent theinsulating gasket 91 interposed between the foldedportion 98 of themetal case 83 and the curved side surfaces of the sealingplate 94 frombeing compressed excessively, making it possible to prevent the sealingplate 94 from being deformed and, thus, to improve the air-tightness ofthe secondary cell.

What should also be noted is that, in the present invention, the openend portion of the metal case 83 is inwardly folded to conform with thecurvedside surfaces of the sealing plate 94, as described above. Theparticular folding of the open end portion of the metal case 83 makes itpossible to suppress the bending of the longer sides of the open endportion of the metal case 83, the bending being possibly caused by thepressure of the gas generated within the metal case 83 by, for example,short-circuiting or over-charging of the secondary cell. It follows thatit is possible to maintain a high air-tightness in the event of thepressure increase withinthe metal case 83. Incidentally, the seconddrawing mold 75 and the second curling mold 80 also produce the effectssimilar to those produced by the first drawing mold 63 and the firstcurling mold 69.

As a matter of fact, the secondary cell prepared in Example 3 was foundto exhibit excellent properties, when compared with Comparative Example2 which follows.

COMPARATIVE EXAMPLE 2

A secondary cell was manufactured as in Example 3, except that used inComparative Example 2 was a drawing mold, in which the inner surface ofthe hollow was flat, and a curling mold, in which the inner surface oftherecess was flat. In Comparative Example 2, the sealing plate of thesecondary cell was excessively pushed by the insulating gasket in thesealing process so as to give rise to deformation by warping.Incidentally, the secondary cell in each of Example 3 and ComparativeExample 2 was not provided with an explosion-preventing mechanism forthe pressure resistance test described below.

Specifically, 10 secondary cell samples were prepared for each ofExample 3and Comparative Example 2. A pressure boosting device wasmounted to the main body portion of the metal case in each of thesecondary cell samples so as to measure the gas pressure at which a gasleaks from the sealing portion of the secondary cell to the outside. Theparticular gas pressure was measured by a pressure sensor, with theresults as shown in Table 3.

                  TABLE 3                                                         ______________________________________                                                   Gas pressure (kg/cm.sup.2)                                         ______________________________________                                        Example 3    11 to 13                                                         Comparative                                                                   Example 2     2 to 11                                                         ______________________________________                                    

As apparent from Table 3, the secondary cell manufactured by the methodof Example 3 was found to exhibit such a high pressure resistance as 11to 13kg/cm². In other words, the secondary cell was found to exhibit amarkedly improved air-tightness. On the other hand, the secondary cellmanufactured by the method of Comparative Example 2 was found to exhibitapressure resistance as low as only 2 to 11 kg/cm². As a matter of fact,warping was recognized in the sealing plate included in the cellmanufactured by the method of Comparative Example 2.

EXAMPLES 4 TO 8

FIG. 20 shows the construction of a rectangular nickel-metal hydridesecondary cell prepared in each of Examples 4 to 8.

In the first step, a stepped portion 100 was formed by outwardlyexpanding the open end portion of a metal case 99, which also acts as anegative electrode terminal and is in the form of a rectangular cylinderhaving a bottom. Then, an electrode group 104 prepared by laminating apositive electrode 102 and a negative electrode 103 one upon the otherwas housed in the metal case 99. The positive electrode 102 noted abovewas held between two parts of a separated 101 which was folded to havethe positiveelectrode 102 wrapped therein and contained nickel hydroxideas an active substance. On the other hand, the negative electrode 103contained a hydrogen absorption alloy as an active substance. An alkalielectrolyte was contained in the metal case 99.

In the next step, a sealing lid group 107, which performs theexplosion-preventing function and acts as a positive electrode terminal,was mounted on an insulating gasket 106, which had a bottom, wasrectangular in cross section, and had a rectangular hole 105 formed inthebottom. The sealing lid group 107 noted above comprised a sealingplate 109having a gas releasing hole 108 formed in the central portion,an elastic valve body 110 made of, for example, a synthetic rubber, anda hat-shaped terminal cap 111 having a gas releasing hole (not shown)formed therein. The elastic valve body 110 is disposed on the sealingplate 109 to close the gas releasing hole 108. The terminal cap 111 isarranged to surround the elastic valve body 110 and is welded to thesealing plate 109.

After the sealing lid group 107 was mounted on the insulating gasket106, apositive electrode lead 112 having one end connected to thepositive electrode 102 was connected at the other to the lower surfaceof the sealing plate 109. Then, the insulating gasket 106 having thesealing lid group 107 mounted thereon was mounted on the stepped portion100 of the metal case 99. Further, the sealing lid group 107 washermetically fixed to the metal case 99 with the insulating gasket 106interposed therebetween.

In the next step, the secondary cell assembled as described above wascharged by 150% at 0.1 CmA, followed by applying the discharging and,then, aging treatment to the secondary cell under the conditions givenin Table 4 below so as to activate the negative electrode and, thus, toprepare a secondary cell having a capacity of 600 mAh. Incidentally, theresidual capacity in the discharging step, which is shown in Table 4,denotes the ratio of the residual capacity to the entire dischargecapacity of the secondary cell.

                  TABLE 4                                                         ______________________________________                                                                Conditions                                            Discharge conditions before aging                                                                     of aging                                              Discharge     Cut-Off  Residual Temper-                                       current       Voltage  capacity ature  Time                                   (Cm A)        (V)      (%)      (°C.)                                                                         (Hr)                                   ______________________________________                                        Example 4                                                                             1         1.0      20     25     72                                   Example 5                                                                             1         1.0      20     45     24                                   Example 6                                                                             1         0.8      13     25     72                                   Example 7                                                                             1         0.8      13     45     24                                   Example 8                                                                             0.2       1.0       4     25     72                                   ______________________________________                                    

COMPARATIVE EXAMPLES 3 AND 4

Secondary cells were prepared as in Examples 4 to 8, except that theassembled secondary cells were subjected to the discharge and agingtreatments under the conditions shown in Table 5:

                  TABLE 5                                                         ______________________________________                                                              Conditions                                              Discharge conditions before aging                                                                     of aging                                              Discharge     Cut-Off  Residual Tempara-                                      current       voltage  capacity ture   Time                                   (Cm A)        (V)      (%)      (°C.)                                                                         (Hr)                                   ______________________________________                                        Compara-                                                                              1         1.0      20     Not    Not                                  tive                              per-   per-                                 Example 3                         formed formed                               Compara-                                                                              1         1.1      29     25     72                                   tive                                                                          Example 4                                                                     ______________________________________                                    

A capacity screening test was applied to each of the secondary cellsprepared in Examples 4 to 8 and Comparative Examples 3 and 4 by chargingthe secondary cell by 130% at 1 CmA, followed by discharging the cell to1.0 v with 1 CmA so as to measure the discharge capacity. Then, the cellwas charged again by 150% with 0.2 C so as to measure the dischargecapacity and the average discharge voltage in the case of dischargingthe cell at 1 C with the cut-off voltage set at 1 V.

FIG. 21 shows the result in respect of the discharge capacity ratio interms of the discharge capacity at the time of the capacity screeninginspection and the discharge capacity at the first charging-dischargingcycle after the capacity screening inspection. The discharge capacity atthe screening inspection of the secondary cell prepared in Example 4 wasset at 100 in determining the data plotted in the graph of FIG. 21. Ontheother hand, Table 6 below shows the average discharge voltage in thefirst charging-discharging cycle after the capacity screeninginspection:

                  TABLE 6                                                         ______________________________________                                                      Average discharge voltage (V)                                   ______________________________________                                        Example 4       1.1927                                                        Example 5       1.1999                                                        Example 6       1.1948                                                        Example 7       1.2050                                                        Example 8       1.1960                                                        Comparative Example 3                                                                         1.1764                                                        Comparative Example 4                                                                         1.1795                                                        ______________________________________                                    

FIG. 21 and Table 6 collectively show that the secondary cells preparedin Examples 4 to 8 exhibit sufficiently improved discharge capacity andaverage discharge voltage in each of the screening inspection time andthefirst charging-discharging cycle after the screening inspection time.What should be noted is that the secondary cell prepared in the Examplesof thepresent invention permits decreasing the difference in thedischarge capacity between the screening inspection time and the firstcharging-discharging cycle after the screening inspection time. Thisindicates that the screening inspection is carried out with a highaccuracy in the present invention.

When it comes to the secondary cell prepared in each of ComparativeExamples 3 and 4, however, the activation degree was insufficient,leadingto very low discharge capacity and the average discharge voltagein the screening inspection time. Also, since the difference in thedischarge capacity was large between the screening inspection time andthe first charging-discharging cycle after the screening inspectiontime, the accuracy of the screening inspection tends to be lowered.

Further, charging-discharging cycles were applied 90 times to each ofthe secondary cells prepared in Examples 4 to 8 and Comparative Examples3 and4 so as to measure the change in the discharge capacity. In each ofthe charging-discharging cycle, the secondary cell was charged first by125% with 1C, followed by discharging the charged secondary cell with1C, with the cut-off voltage set at 1 V. FIG. 22 is a graph showing theresults of the test. To be more specific, the data changing in thedischarge capacityratio relative to the number of charging-dischargingcycles were plotted inthe graph of FIG. 22. The discharge capacity atthe screening inspection ofthe secondary cell prepared in Example 1 wasset at 100 in determining the data plotted in the graph of FIG. 22.

FIG. 22 clearly shows that the secondary cell prepared in each ofExamples 4 to 8 permits maintaining a high discharge capacity ratio overthe entireperiod of the test starting with the initial stage of thecharging-discharging cycle test. It is seen from FIG. 22 that theparticular tendency remains unchanged even after completion of the 90thcycle. When it comes to each of the secondary cells prepared inComparative Examples 3 and 4, however, the discharge capacity ratio intheinitial stage of the charging-discharging cycles was markedly lowerthan that of the secondary cells prepared in Examples 4 to 8. As amatter of fact, the discharge capacity ratio in Comparative Examples 3and 4 can certainly be improved after 90th charging-discharging cycle toa level substantially equal to that in the starting time of thecharging-discharging cycle for the secondary cells prepared in Examples4 to 8. However, a marked difference is recognized in the dischargecapacityratio between the initial stage of the charging-dischargingcycles and the completion time of 90th charging-discharging cycle. Inother words, the data for Comparative Examples 3 and 4 are markedlyincreased with increasein the number of charging-discharging cycles, asshown in FIG. 22.

The current value employed in the charging-discharging step in Examples4 to 8 need not be restricted to that described previously. For example,it is also possible to employ a step charging-discharging operation inwhich the current value is changed stepwise.

As described above in detail, the present invention provides arectangular nickel-metal hydride secondary cell which permits preventingthe open end portion of a metal case from being deformed when the openend portion noted above is folded inward and which also permitsmaintaining a high air-tightness.

The present invention also provides a method of manufacturing arectangularnickel-metal hydride secondary cell, which permits foldingthe open end portion of a metal case after a diameter-diminishingtreatment in a desired shape, which permits easily detaching a curlingmold from the metal case, and which also permits preventing reduction ofreliability which would be caused by, for example, the warping of thesealing plate.

The present invention also provides a method of manufacturing arectangularnickel-metal hydride secondary cell, which permits preventinga sealing plate from being deformed in the sealing process and alsopermits improving the air-tightness of the secondary cell.

Further, the present invention provides a method of manufacturing arectangular nickel-metal hydride secondary cell, which permitssufficiently improving the discharge capacity and the discharge voltagebefore the screening inspection so as to improve the accuracy of thescreening inspection.

Additional advantages and modifications will readily occur to thoseskilledin the art. Therefore, the invention in its broader aspects isnot limited to the specific details, and illustrated examples shown anddescribed herein. Accordingly, various modifications may be made withoutdeparting from the spirit or scope of the general inventive concept asdefined by the appended claims and their equivalents.

What is claimed is:
 1. A method of manufacturing a rectangularnickel-metal hydride secondary cell, comprising the steps of:housing anelectrode group and an alkali electrolyte in a rectangular cylindricalmetal case having a bottom and a stepped portion formed along the innersurface of said metal case by enlarging the open end portion of saidmetal case, said electrode group being constructed such that a nickelpositive electrode and a hydrogen absorption alloy negative electrodeare superposed one upon the other with a separator interposed betweenthe adjacent positive and negative electrodes; mounting a rectangularcylindrical insulating gasket having a bottom and a rectangular sealingplate housed therein in advance on the stepped portion of said metalcase, a rectangular hole being formed in the bottom of said insulatinggasket; reducing the diameter of the open end portion of said metal caseto a value substantially equal to that of the main body portion of saidmetal case by inserting the open end portion of said metal case to adrawing mold so as to press inward the rising wall of the insulatinggasket, the mold having a rectangular hollow in the central portion anda tapered portion formed by outwardly enlarging the inner surface of thehollow in the lower portion of said mold and abutting a curling moldagainst the open end portion of said metal case by dropping said curlingmold to said metal case, so as to inwardly fold the open end portion ofsaid metal case and, thus, to permit said sealing plate to be fixedunder pressure to said metal case with said insulating gasket interposedtherebetween, said curling mold having a rectangular hollow in thecentral portion, a rectangular recess formed in the bottom portion tocommunicate with the hollow and sized larger than the hollow, and atapered portion formed in the inner surface in the lower end portion ofthe recess, the tapered portion having an angle of 0.5° to 10°, risingsaid curling mold while holding downward said sealing plate withknock-out vertically movable within the hollow of said curling mold soas to detach said curling mold from said metal case.
 2. The methodaccording to claim 1, wherein the curvature radius r₁ in the cornerportion of the hollow included in said drawing mold is greater than thecurvature radius r₂ in the corner portion of the recess included in saidcurling mold.
 3. The method according to claim 1, wherein the taperedportion of said drawing mold has an angle of 1° to 5°.
 4. The methodaccording to claim 1, wherein said insulating gasket is made of apolyamide series resin containing carbon black or graphite.
 5. A methodof manufacturing a rectangular nickel-metal hydride secondary cell,comprising the steps of:housing an electrode group and an alkalielectrolyte in a rectangular cylindrical metal case having a bottom anda stepped portion along the inner surface of said metal case formed byenlarging the open end portion of said metal case, said electrode groupbeing constructed such that a nickel positive electrode and a hydrogenabsorption alloy negative electrode are superposed one upon the otherwith a separator interposed between the adjacent positive and negativeelectrodes; mounting a rectangular cylindrical insulating gasket havinga bottom and a rectangular sealing plate housed therein in advance onthe stepped portion of said metal case, a rectangular hole being formedin the bottom of said insulating gasket, and the sealing plate havinglonger side surfaces which are curved outward; reducing the diameter ofthe open end portion of said metal case to a value substantially equalto that of the main body portion of said metal case by using a drawingmold so as to press inward the rising wall of said insulating gasket,said mold having a rectangular hollow in the central portion and atapered portion formed by outwardly enlarging the inner surface of thehollow in the lower portion of said mold; and abutting a curling moldagainst the open end portion of said metal case, so as to inwardly foldthe open end portion of said metal case to form a folded portion and,thus, to permit said sealing plate to be fixed under pressure to saidmetal case with said insulating gasket interposed therebetween, saidcurling mold having a rectangular hollow in the central portion, arectangular recess formed in the bottom portion to communicate with thehollow and sized larger than the hollow, and a tapered portion formed inthe inner surface in the lower end portion of the recess; wherein thehollow of said drawing mold has outwardly curved inner surfaces in theregions corresponding to the curved side surfaces of said sealing plate,and the recess included in said curling mold has outwardly curved innersurfaces in the regions corresponding to the curved side surfaces ofsaid sealing plate.
 6. The method according to claim 5, wherein thewidth in the central portion of the curved inner surfaces in the hollowof said drawing mold is larger by at least 0.5% than the width in theedge portion of the curved inner surfaces.
 7. The method according toclaim 5, wherein the width in the central portion of the the curvedinner surfaces in the recess of said curling mold is larger by at least0.5% than the width in the edge portion of the curved inner surfaces.